1. Field of the Invention
The present invention relates generally to a rotary drilling arrangement for mitigating pressure-differential sticking of a drill string in a wellbore. More particularly, the subject invention concerns a method and apparatus for drilling deviated well bores, such as in extended reach drilling, which are particularly designed to reduce the chance of pressure-differential sticking of the drill string by pumping the drilling cuttings from the wellbore bottom by reverse circulation using a pump powered by the cones of the rotary bit.
Extended Reach Drilling is concerned with rotary drilling procedures to drill, log and complete wellbores at significantly greater inclinations and/or over horizontal distances substantially greater than currently being achieved by conventional directional drilling practices. The success of extended reach drilling should benefit mainly offshore drilling projects as platform costs are a major factor in most offshore production operations. Extended reach drilling offers significant potential for (1) developing offshore reservoirs not otherwise considered to be economical, (2) taping sections of reservoirs presently considered beyond economical or technological reach, (3) accelerating production by longer intervals in the producing formation due to the high angle holes, (4) requiring fewer platforms to develop large reservoirs, (5) providing an alternative for some subsea completions, and (6) drilling under shipping fairways or to other areas presently unreachable.
A number of problems are presented by high angle extended reach directional drilling. In greater particulrity, hole inclinations of 60.degree. or greater, combined with long sections of hole or complex wellbore profiles present significant problems which need to be overcome in extended reach drilling. The force of gravity, coefficients of friction, and mud particle settling are the major physical phenomena of concern.
As inclination increases, the available weight from gravity to move the pipe or wireline string down the hole decreases as the cosine of the inclination angle, and the weight lying against the low side of the hole increases as the sine of the inclination angle. The force resisting the movement of the drill string is the product of the apparent coefficient of friction and the sum of the forces pressing the string against the wall. At an apparent coefficient of friction of approximately 0.58 for a common water base mud, drill strings tend to slide into the hole at inclination angles up to approximately 60.degree.. At higher inclination angles, the drill strings will now lower from the force of gravity alone, and must be mechanically pushed or pulled, or alternatively the coefficient of friction can be reduced. Since logging wirelines cannot be pushed, conventional wireline logging is one of the first functions to encounter difficulties in this type of operation.
Hole cleaning also becomes a more significant problem in high angle bore holes because particles need fall only a few inches to be out of the mud flow stream and to come to rest on the low side of the hole, usually in a flow-shaded area alongside the pipe. This problem is also encountered in substantially vertical wellbores but the problem is much worse in deviated wellbores. In deviated wellbores the drill string tends to lie on the lower side of the wellbore and drill cuttings tend to settle and accumulate along the lower side of the wellbore about the drill string. This condition of having drill cuttings lying along the lower side of the wellbore about the drill string along with the usual filter cake on the wellbore wall presents conditions susceptible for differential sticking of the drill pipe when a porous formation is penetrated that has internal pressures less than the pressures existing in the borehole.
Cuttings generated by rock bits are usually less than 1/2" in size and are usually plate-like in structure. A second source of cuttings, which are not really cuttings from the bit, are those generated by sloughing or by erosion of the borehole wall, and these are frequently 1" to 11/2" in length and thicker than a drilled cutting. In general, the larger the cutting size, the more difficult it is to transport it in the mud stream. In mitigation of this, it should be pointed out that some regrinding of the cuttings normally takes place in all rotary-drilled holes by the drill string, particularly the drill collars, by crushing between the rotating pipe and the wall of the hole.
This settling of cuttings is particularly significant in the near horizontal holes expected to be drilled in extended reach drilling. Present drill strings of drill pipe body, tool joints and drill collars are usually round and rotate concentrically about a common axis. If the pipe rotates concentrically around the same axis as the tool joints which are normally positioned against the solid wall and act as bearings for the rotating string, then a long "keyseat" is developed as the pipe is buried and beds itself into the cuttings and wall cake. A similar action of a drill string rotating about a concentric axis in a thick wall cake in a vertical hole could produce the same results. If differential pressure (borehole mud pressure less formation pore pressure) exists opposite a permeable zone in the formation, then conditions are set for the pipe to become differentially wall stuck. In both cases, the pipe is partially buried and bedded into a mass of solids, and can be hydraulically sealed to such an extent that there is a substantial pressure difference in the interface of the pipe and the wall and the space in the open borehole. This hydraulic seal provides an area on the pipe for the pressure differential to force the pipe hard against the wall. The frictional resistance to movement of the pipe against the wall causes the pipe to become immovable, and the pipe is in a state which is commonly referred to as differentially stuck.
2. Discussion of the Prior Art
Pressure-differential sticking of a drill pipe is also discussed in a paper entitled "Pressure-Differential Sticking of Drill Pipe and How It Can Be Avoided or Relieved" by W. E. Helmick and A. J. Longley, presented at the Spring Meeting of the Pacific Coast District, Division of Production, Los Angeles, Calif., in May 1957. This paper states that the theory of pressure-differential sticking was first suggested when it was noted that spotting of oil would free pipe that had stuck while remaining motionless opposite a permeable bed. This was particularly noticeable in a field wherein a depleted zone at 4300 feet with a pressure gradient of 0.035 psi per foot was penetrated by directional holes with mud having hydrostatic gradients of 0.52 psi per foot. In view thereof, it was concluded that the drill collars lay against the filter cake on the low side of the hole, and that the pressure differential acted against the area of the pipe in contact with the isolated cake with sufficient force that a direct pull could not effect release. This paper notes that methods of effecting the release of such a pipe include the use of spotting oil to wet the pipe, thereby relieving the differential pressure, or the step of washing with water to lower the pressure differential by reducing the hydrostatic head. Field application of the principles found in a study discussed in this paper demonstrate that the best manner for dealing with differential sticking is to prevent it by the use of drill collar stabilizers or, more importantly, by intentionally shortening the intervals of time when pipe is at rest opposite permeable formations.
Drilling fluid or mud used in rotary drilling of wellbores is usually a mixture of water, clay, weighting material, and a few chemicals. Sometimes oil may be used instead of water, or a little oil is added to the water to give the mud certain desirable properties. Drilling mud serves several very important functions including the following. Mud is used to raise the cuttings made by the bit and lift them to the surface for disposal. Equally important, the mud also provides a means for keeping underground pressures in check. Since a hole full of drilling mud exerts pressure, the mud pressure can be used to contain pressure in a formation. Clay is frequently added to the mud so that it can keep the bit cuttings in suspension as they move up the hole. The clay also sheaths the wall of the hole. This thin veneer of clay is termed wall cake, and makes the hole stable so it will not cave in or slough.
The equipment in a typical drilling fluid circulating system consists of a large number of components. A mud pump takes in mud from the mud pits and sends it out a discharge line to a standpipe. The standpipe is a steel pipe mounted vertically on one leg of the mast or derrick. The mud is pumped up the standpipe and into a flexible, very strong, reinforced rubber hose called the rotary hose, or kelly hose. The rotary hose is connected to the swivel. The mud enters the swivel, flows down the kelly, drill pipe, and drill collars, and exists at the bit. It then flows with a sharp U-turn and heads back up the hole in the annulus which is the space between the outside of the drill string and wall of the hole.
Finally, the mud leaves the hole through a steel pipe called the mud-return line and falls over a vibrating, screenlike device called the shale shaker. The shaker screens out the cuttings and dumps them into one of the reserve pits (the earthen pits excavated when the site was being prepared). Accordingly the circulating system is essentially a closed loop system. The mud is circulated over and over again throughout the drilling of the well.
However, the prior art has also recognized that there may exist special circumstances under which a reverse circulation of drilling fluid is desirable, and in which the fluid flows in a reverse manner to that described above, namely down the annulus and up the drill string. The following U.S. patents disclose an approach of that nature, Henderson U.S. Pat. No. Re. 26,669, Wells U.S. Pat. No. 2,786,652, Failing U.S. Pat. No. 2,849,213, Baud et al. U.S. Pat. No. 3,012,621, Schumacher, Jr. U.S. Pat. No. 3,292,719, Elenburg U.S. Pat. No. 3,416,617, Kunnemann U.S. Pat. No. 3,416,618, Elenburg U.S. Pat. No. 3,419,092, Cobbs et al. U.S. Pat. No. 3,958,650, Young U.S. Pat. No. 3,958,651, and Wallers U.S. Pat. No. 4,055,224.
Moreover, a further area of the prior art is somewhat related to the present invention by their disclosures on drilling fluid pumps powered by the rotating roller cones of a drill bit, and these include Evans et al. U.S. Pat. No. 1,572,274, Sloan U.S. Pat. No. 1,656,798, Krall U.S. Pat. No. 2,056,471, Reynolds U.S. Pat. No. 2,646,128, Haines U.S. Pat. No. 3,384,179 and Vida et al. U.S. Pat. No. 3,736,994.
However, none of the prior art discussed and cited above is concerned with mitigating differential sticking of a drill string in a highly deviated wellbore by pumping the drilling cuttings from the wellbore bottom by reverse circulating drilling fluid with a pump which is mechanically driven by the rotating roller cones of the drill bit.